Well I normally stick to water. I think that diet drinks taste horribly sweet.
Lot's of times there are no diet drinks anyhow, yesterday at a village lunch I could have had orange juice, muscat(a very sweet wine) or Ricard for an aperitif. I stuck to water (does make you look a bit odd, using the water that is meant to go in the Ricard to drinK)
I find it has the opposite effect. If I drink a fizzy drink before a meal I wind up not eating as much because I'm too full from the soda! (sparkling water doesn't have the same effect, though.)